With a number of devices connected together over a bus, it is often useful to know the identities of each device. The particular identity of a device can be useful to a host that has to send data to that device. With knowledge of the device, the host can load the appropriate device driver for interacting with that device.
On a 1394 bus, also known as Firewire, a reset signal on the bus initiates a tree identification and self identification of all nodes on the bus. The tree identification process distributes to the nodes the connectedness between the various nodes. It indicates which ports are connected to which other ports and labels each node as a parent or child. In the self-identification process, each node has an opportunity to select a unique physical I.D. for building a system topology map. For example, each node will receive a number from 0 to N−1, where N is the number of nodes on the bus. In accordance with the self-identification process on the 1394 bus, the root node identified during tree identification passes control to its lowest numbered connected port and waits for that node to finish the identification of each of the child nodes on that port. The root node then passes control to the next highest port and waits for the nodes on that port to finish. When the nodes attached to all ports are finished, the root node itself does a self-identify. Each of the child nodes on the branches does the self-identification process with respect to its own child nodes in the same way as the root node.
A node doing a self identify sends its physical I.D. information out over the bus. The physical I.D. selected by a node is simply the count of the number of times the node has passed through the state of receiving self I.D. information from others before having its own opportunity to send self I.D. information.
As a result of the tree identification and self identification processes, the topology is determined and communicated to each of the connected nodes. The information communicated to each of these nodes consists of the physical I.D. of every node and its port connectedness information, i.e., whether a node's port is a parent port, a child port, an unconnected port or unimplemented port. The topology describes a tree of connections between the ports of the various nodes on the bus. There has been no communication with regard to the actual identity of the device at each node so that an appropriate device driver could be selected and loaded. The actual identity is indicated by the global unique identifier and associated identifying information regarding the device at that node.
Currently, a host occupying one of the nodes interested in learning the unique identification of each of the devices on the bus performs a process on the bus to learn the identities of each of the devices at the nodes. The host reads the global unique identifier of each device on the bus using bus transactions. The global identifiers thus obtained are compared with the global identifiers that were present on the bus prior to the reset. Where a match is found for the global unique identifier, the rest of the identifying information for that node can be assigned to the appropriate node on the new tree. Where there is no match for a global unique identifier on the new tree, further discovery over the bus must be conducted to obtain the remaining identifying information from the configuration ROMs of the new devices. As can be seen, when there are several host systems on the bus, each wanting simultaneously to identify the devices in this manner, there will be many more bus transactions seeking the necessary information.